Large arrays of solar panels have been installed for utility-scale electric power generation in several locations around the world. For example, an array of solar panels in Bavaria, Germany comprising more than 57,000 photovoltaic (PV) modules covers an area of almost 62 acres (25 square kilometers) and generates approximately 10 megawatts of electrical power. Another solar power system in Nevada in the U.S. will have approximately 70,000 PV modules over an area of about 140 acres (57 square kilometers). As the efficiency and reliability of solar panels increase and installed costs decrease, large arrays of solar panels are expected to become more common sources of electrical power.
Some solar panels include an energy conversion apparatus for converting sunlight into another form of energy. Other panels comprise reflective optical elements, transmissive optical elements, or a combination of transmissive and reflective elements to concentrate incident sunlight onto an energy conversion apparatus, which may alternatively be attached to the solar panel or be separated from the solar panel. For example, a solar panel may include one or more PV modules for converting sunlight into electricity. A PV module comprises many relatively small solar cells connected together in a series electrical circuit. In bright sunlight, a PV module may output up to a few hundred watts of direct-current (DC) electrical power at a voltage from about 12 volts DC to about 50 volts DC, depending on the panel design. The output from an individual PV module may be combined with the outputs from other PV modules in a series electrical circuit for efficient power transmission.
A solar panel having one or more PV modules may further include optical elements for concentrating sunlight incident upon a relatively large surface of the solar panel onto a solar cell having a relatively small surface, or alternatively onto a plurality of solar cells electrically connected in series. Such a PV module, referred to as a concentrating photovoltaic module (CPV), may operate with higher energy efficiency than a system without optical elements for concentrating sunlight. An example of a CPV module is shown in the prior-art illustration of FIG. 6. In FIG. 6, a pictorial view of a small segment of a first example of a CPV module 600-1 includes a plurality of lenses 602 arranged in rows and columns above a plurality of solar cells 604 attached to a substrate 608. The substrate 608 provides structural support for other components in the CPV module 600-1 and may further include structures for dissipating heat. A solar cell 604 is positioned close to a focal point for a lens 602. Adjacent solar cells are connected in series with an electrical conductor 606.
Another example of a CPV module is shown in the prior-art illustration of FIG. 7. FIG. 7 is a partial end view of a CPV module 600-2 comprising a reflector 610 which directs incident sunlight onto a surface of a solar cell 604 positioned close to an optical focus of the reflector 610. The solar cell 604 is attached to a substrate 608 for structural support. Some CPV modules include a plurality of solar cells 604, each one dispositioned near the optical focus of a corresponding plurality of cup-shaped reflectors 610. Other CPV include a plurality of solar cells 604 dispositioned along an optical focus of a trough-shaped reflector 610. Many variations of lens and reflector designs are found in CPV modules known in the art.
Directing too much sunlight onto a solar cell may damage the solar cell by overheating it. Therefore, a PV module may include a device for measuring the temperature of a solar cell. A PV module may include many temperature measurement devices to detect hot spots in the module. An operator of a solar power system may choose to turn a solar panel away from the sun to lower the temperature of a PV module in which a hot spot is detected. CPV modules in particular must be monitored for hot spots since sunlight collected from a relatively large collecting surface is focused onto a relatively small solar cell. In the prior-art illustrations of FIG. 6 and FIG. 7, a temperature measurement device 122 is thermally coupled to a solar cell 604. Temperature sensors may be attached to many solar cells as shown. An output signal from the temperature measurement device is monitored by a system operator to detect a temperature above a safe operating limit in a solar cell 604. In a large solar panel, it may be necessary to monitor temperatures at many locations within a PV module.
In order to increase power generation, a mechanical positioning system may be used to rotate a solar panel in azimuth and elevation to track the sun along its path across the daytime sky. For example, a solar panel having an attached PV module may be rotated so that a sunlight-absorbing surface of the PV module is perpendicular to rays of sunlight throughout daylight hours. Or, a solar panel comprising one or more mirrors may be rotated so as to direct sunlight onto a separate PV module which remains in a fixed position relative to the sun. A solar panel adapted to track the sun's position and direct reflected sunlight onto a target is referred to as a heliostat. In some solar power systems, a PV module receives sunlight reflected from more than one heliostat.
Measured values of azimuth angle and elevation angle may be transmitted from a solar panel through a communications network to a central location for review by a management and control system and a system operator. Other parameters related to solar panel efficiency, operating conditions, and fault conditions, for example temperatures measured for solar cells in a PV module, may also be sent to the central management and control system, referred to as a central server. Commands from the central server may also be sent over the communications network to equipment connected to or located near a solar panel, for example position adjustment commands sent to a mechanical positioning system.
A plurality of serial chains of solar panels connected in parallel for input to an inverter and a power transformer is referred to as an area. The solar panels within one area are generally located in relatively close physical proximity to one another. A solar power plant may comprise several areas to achieve a preferred electrical power generating capacity. Large solar power plants occupy a substantial expanse of land, as in the examples of solar power plants in Bavaria and Nevada. For solar power plants comprising many areas, areas that have many solar panels each, or where there is a substantial distance separating one area from another, each area may have its own server, referred to as a gateway, for accumulating data from solar panels in an area and transmitting accumulated data to the central server. A gateway may also distribute commands from the central server to solar panels in an area served by the gateway.
In order to exchange data and commands with a central server and one or more gateways, a solar panel is part of a node in a communications network linking nodes to other nodes, nodes to a gateway, and gateways to a central computer. As the number of nodes increases, the cost of establishing reliable communications connections between the nodes increases. Furthermore, as the number of components used in the communications network increases, the reliability of the network decreases. While there are many different network topologies for communications related to monitoring and control known in the art, none of the known topologies are optimum for monitoring and control of large numbers of solar panels distributed over large expanses of land. For example, connecting solar panels to each other and to a gateway in a solar panel array covering many acres (thousands of square meters) with point-to-point wiring such as electrical cable or optical fiber is very expensive to install and maintain. Such wiring must be protected from mechanical damage, for example damage from service vehicles driving over the wiring, and may further need to be isolated from sources of electrical interference such as inverters, transformers, transmission cables, electrical storms, electrical switchgear, vehicle ignition systems, computer systems, and so on.
Another method known in the art for establishing communications between nodes in a control and monitoring network is referred to as power line communications (PLC). PLC technology includes interface circuits for coupling data to be transmitted from a data source onto an electrical power transmission line. Transmitted data is decoupled from the power transmission line at a data destination by a receiving circuit that isolates the equipment receiving the data from damage by voltage and current on the power transmission line. An advantage of a PLC system is that separate wiring is not required for power transmission and communications. However, communications over a PLC system may be degraded or disrupted by some of the same sources of electrical interference described for point-to-point wiring solutions.
Wireless technology is another widely used method for establishing communications links between nodes in a network. Both short range and long range wireless communications technologies, for example the short range technology popularly referred to as “Bluetooth” and the longer range technology popularly referred to as “Wifi”, may be adapted to exchange data and commands between nodes and a central server. However, intervening terrain or buildings, temporary obstructions such as service equipment, solar panels or their metal support structures, and sources of electrical interference, some of which have already been noted, may interfere with wireless communications. Furthermore, wireless technologies are generally not able to be expanded to include thousands of nodes within range of a single wireless network access point, for example a gateway communicating with hundreds or thousands of solar panels in an area in a solar power plant. It may be possible to increase the number of wireless access points to add capacity for more nodes, but because of the close proximity of large numbers of panels, the wireless access points may then be close enough to interfere with each other. Preventing such interference by, for example, assigning different frequencies to different wireless access points may still result in a maximum number of wireless connections that is less than the number of nodes needed for a large solar panel array.
Each networking technology known in the art has limitations that either reduce scalability to large numbers of nodes or which may result in network communications errors under conditions which may be expected to occur during the operation of a large solar panel array. For example, should any one panel in a serially connected chain of panels fail, for example by the failure of a solar cell within a PV module or by a failure in an electrical connection between solar panels, not only may output from the failed panel be interrupted, but output from other panels preceding the failed panel in the serial chain may be interrupted. Also, communication of data and commands may be disrupted, for example data and commands carried in a PLC system over electrical power connections between nodes. Loss of network communications interrupts monitoring and control of the solar panel array and may also result in an interruption of electrical power service from a solar power plant.
What is needed is a network topology for controlling and monitoring solar panels that is scalable from arrays comprising a few solar panels to arrays comprising hundreds of thousands of solar panels. What is also needed is a network topology that includes redundant communications pathways so that interruption of a communications pathway through a node does not result in a loss of monitoring or control of other nodes. What is further needed is a network topology that is able to bypass solar panels that have malfunctioned and maintain electric power flow from and communications with other solar panels.